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ESET researchers publish a white paper placing IIS internet server threats below the microscope
ESET researchers have found a set of beforehand undocumented malware households, applied as malicious extensions for Web Info Providers (IIS) internet server software program. Concentrating on each authorities mailboxes and e-commerce transactions, in addition to aiding in malware distribution, this various class of threats operates by eavesdropping on and tampering with the server’s communications.
Together with a whole breakdown of the newly found households, our new paper, Anatomy of native IIS malware, gives a complete information to assist fellow safety researchers and defenders detect, dissect and mitigate this class of server-side threats. On this blogpost, we summarize the findings of the white paper.
Right now, we’re additionally launching a sequence of blogposts the place we introduce essentially the most notable of the newly found IIS malware households, as case research of how the sort of malware is used for cybercrime, cyberespionage and SEO fraud.
The findings of our IIS malware analysis had been first offered at Black Hat USA 2021 and also will be shared with the group on the Virus Bulletin 2021 convention on October 8th.
IIS is Microsoft Home windows internet server software program with an extensible, modular structure that, since v7.0, helps two kinds of extensions – native (C++ DLL) and managed (.NET meeting) modules. Specializing in malicious native IIS modules, we have now discovered over 80 distinctive samples used within the wild and categorized them into 14 malware households – 10 of which had been beforehand undocumented. ESET safety options detect these households as Win{32,64}/BadIIS and Win{32,64}/Spy.IISniff.
How IIS malware operates
IIS malware is a various class of threats used for cybercrime, cyberespionage, and web optimization fraud – however in all circumstances, its important objective is to intercept HTTP requests incoming to the compromised IIS server and have an effect on how the server responds to (a few of) these requests.
With the default set up, IIS itself is persistent, so there is no such thing as a want for extension-based IIS malware to implement extra persistence mechanisms. As soon as configured as an IIS extension, the malicious IIS module is loaded by the IIS Employee Course of (w3wp.exe), which handles requests despatched to the server – that is the place IIS malware can intrude with the request processing.
We recognized 5 important modes wherein IIS malware operates, as illustrated in Determine 1:
- IIS backdoors enable their operators to remotely management the compromised laptop with IIS put in
- IIS infostealers enable their operators to intercept common visitors between the compromised server and its legit guests, to steal data equivalent to login credentials and cost data. Utilizing HTTPS doesn’t forestall this assault, as IIS malware can entry all information dealt with by the server – which is the place the information is processed in its unencrypted state.
- IIS injectors modify HTTP responses despatched to legit guests to serve malicious content material
- IIS proxies flip the compromised server into an unwitting a part of the C&C infrastructure for one more malware household, and misuse the IIS server to relay communication between victims of that malware and the actual C&C server
- web optimization fraud IIS malware modifies the content material served to search engines like google to govern SERP algorithms and increase the rating for different web sites of curiosity to the attackers
All of those malware sorts are mentioned at size within the paper.
How (and the place) it spreads
Native IIS modules have unrestricted entry to any useful resource out there to the server employee course of – thus, administrative rights are required to put in native IIS malware. This significantly narrows down the choices for the preliminary assault vector. We’ve got seen proof for 2 eventualities:
- IIS malware spreading as a trojanized model of a legit IIS module
- IIS malware spreading by server exploitation
For instance, between March and June 2021, we detected a wave of IIS backdoors unfold by way of the Microsoft Alternate pre-authentication RCE vulnerability chain (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065), aka ProxyLogon. Focused particularly had been Alternate servers which have Outlook on the internet (aka OWA) enabled – as IIS is used to implement OWA, these had been a very attention-grabbing goal for espionage.
After our colleagues reported the primary such case in March 2021, we have now detected 4 extra campaigns of assorted IIS backdoors spreading to Microsoft Alternate servers by the identical vulnerability. To enhance our telemetry, we have now carried out internet-wide scans to detect the presence of those backdoors, which allowed us to establish and notify different victims of the malware.
Determine 2 reveals the geographical areas of servers affected by these 5 campaigns, utilizing information from our telemetry and internet-wide scans.
The next entities had been among the many victims:
- Authorities establishments in three international locations in Southeast Asia
- A significant telecommunications firm in Cambodia
- A analysis establishment in Vietnam
- Dozens of personal corporations in a variety of industries, positioned largely in Canada, Vietnam and India, and others within the USA, New Zealand, South Korea, and different international locations
Observe that whereas IIS backdoors could also be well-suited for spying on high-profile mailboxes, victims of IIS malware should not restricted to compromised servers – all legit guests of the web sites hosted by these servers are potential targets, because the malware can be utilized to steal delicate information from the guests (IIS infostealers) or serve malicious content material (IIS injectors). Please seek advice from the total white paper for the main points on the targets of the opposite analyzed IIS households.
The insides of native IIS malware
From the technical perspective, all kinds of native IIS malware are applied as dynamic-link libraries (DLLs), written utilizing the IIS C++ API. Any such DLL should:
- Implement a category inherited from both the CHttpModule or CGlobalModule class (or each), and override quite a lot of that class’s strategies (occasion handlers)
- Export the RegisterModule operate, which is the library entry level, accountable for creating the cases of those lessons and registering the applied handlers for server occasions, as illustrated in Determine 3.
Server occasions seek advice from the steps that the IIS server takes throughout request processing (see Determine 4), but in addition to different actions taken by the server (for instance, sending an HTTP response). These occasions generate occasion notifications, that are dealt with by occasion handlers applied within the server’s modules (see Determine 5).
Briefly, the occasion handlers (or the strategies of IIS module core lessons) are the place the IIS malware performance is applied and the place any reverse engineers ought to focus their evaluation. For a deep dive into IIS malware necessities and tips on how to analyze such binaries, seek advice from the Anatomy of native IIS malware part of our white paper.
Community communication
A notable characteristic of IIS malware is the way it communicates with its operators. Malicious IIS modules, particularly IIS backdoors, don’t often create new connections to their C&C servers. They work as passive implants, permitting the attackers to regulate them by offering some “secret” in an HTTP request despatched to the compromised IIS internet server. That’s why IIS backdoors often have a mechanism to acknowledge attacker requests which can be used to regulate the server and have a predefined construction, equivalent to:
- URL or request physique matching a particular regex
- A particular customized HTTP header current
- An embedded token (within the URL, request physique or one of many headers) matching a hardcoded password
- A hash worth of an embedded token matching a hardcoded worth
- A extra complicated situation – for instance, a relationship between all the above
However, some IIS malware classes do implement an alternate C&C channel – utilizing protocols equivalent to HTTP or DNS – to acquire the present configuration on the fly. For instance, an IIS injector contacts its C&C server each time there’s a new request from a legit customer of the compromised web site, and makes use of the server response to change the content material served to that customer (equivalent to malicious code or adware).
Desk 1 summarizes how the C&C channels, in addition to different notable strategies, are applied by the 14 analyzed IIS malware households.
Desk 1. Abstract of obfuscations applied, and functionalities supported by analyzed IIS malware households
Group # | Detection evasion and obfuscation strategies | ||||||||
Backdoor | Infostealer | Proxy | web optimization fraud | Injector | Attacker request verification (e.g. particular header current, particular URI, question string parameter) | Encryption/ encoding |
Various channel protocol | ||
Group 1 | ✅ | ✅ | ❌ | ❌ | ❌ | HTTP header with hardcoded password | base64 | ❌ | ❌ |
Group 2 | ✅ | ❌ | ❌ | ❌ | ❌ | HTTP header with hardcoded password | RSA + AES-CBC | ❌ | ❌ |
Group 3 | ✅ | ❌ | ❌ | ❌ | ❌ | HTTP header current | base64 | ❌ | ❌ |
Group 4 | ✅ | ❌ | ❌ | ❌ | ❌ | HTTP header with hardcoded password | XOR + base64 | ❌ | Anti-logging |
Group 5 | ❌ | ✅ | ❌ | ❌ | ❌ | URI and HTTP header with hardcoded password | ❌ | ❌ | String stacking |
Group 6 | ❌ | ✅ | ❌ | ❌ | ❌ | Question string parameter | ❌ | ❌ | ❌ |
Group 7 | ✅ | ❌ | ❌ | ❌ | ❌ | Relationship between HTTP headers, HTTP physique format | AES-CBC | ❌ | Anti-logging |
Group 8 | ✅ | ❌ | ❌ | ❌ | ❌ | HTTP header with hardcoded password | ❌ | ❌ | ❌ |
Group 9 | ❌ | ❌ | ✅ | ✅ | ❌ | No help for attacker requests | ❌ | HTTP | Encrypted strings (XOR 0x56) |
Group 10 | ❌ | ❌ | ❌ | ✅ | ❌ | No help for attacker requests | ❌ | HTTP to acquire JavaScript config | ❌ |
Group 11 | ✅ | ❌ | ✅ | ✅ | ✅ | HTTP header with hardcoded password | ❌ | DNS TXT to acquire config, HTTP for C&C | String encryption (ADD 0x02) |
Group 12, variant A | ✅ | ❌ | ✅ | ✅ | ✅ | HTTP header with password whose MD5 hash is hardcoded | ❌ | HTTP | String encryption (ADD 0x01) |
Group 12, variant B | ✅ | ❌ | ❌ | ✅ | ✅ | ❌ | HTTP | UPX packing | |
Group 12, variant C | ❌ | ❌ | ❌ | ✅ | ❌ | No help for attacker requests | ❌ | HTTP | String encryption (XOR 0x0C) |
Group 13 | ✅ | ❌ | ❌ | ✅ | ❌ | Question string parameter | ❌ | HTTP | ❌ |
Group 14 | ❌ | ❌ | ❌ | ✅ | ✅ | No help for attacker requests | ❌ | HTTP | ❌ |
Mitigation
Since native IIS modules can solely be put in with administrative privileges, the attackers first must get hold of elevated entry to the IIS server. The next suggestions might assist make their work more durable:
- Use devoted accounts with sturdy, distinctive passwords for the administration of the IIS server. Require multifactor authentication (MFA) for these accounts. Monitor the utilization of those accounts.
- Recurrently patch your OS, and punctiliously think about which companies are uncovered to the web, to scale back the chance of server exploitation.
- Think about using an online software firewall, and/or endpoint safety resolution in your IIS server.
- Native IIS modules have unrestricted entry to any useful resource out there to the server employee course of; you must solely set up native IIS modules from trusted sources to keep away from downloading their trojanized variations. Be particularly conscious of modules promising too-good-to-be-true options equivalent to magically enhancing web optimization.
- Recurrently examine the IIS server configuration to confirm that each one the put in native modules are legit (signed by a trusted supplier, or put in on objective).
For particulars on tips on how to detect and take away IIS malware, seek advice from the Mitigation part of the white paper. We’re additionally publishing a set of YARA rules which you can leverage to detect all of the 14 analyzed IIS malware households.
Conclusion
Web Info Providers internet servers have been focused by varied malicious actors, for cybercrime and cyberespionage alike. The software program’s modular structure, designed to supply extensibility for internet builders, is usually a useful gizmo for attackers to change into part of the IIS server, and intercept or modify its visitors.
It’s nonetheless fairly uncommon for endpoint (and different) safety software program to run on IIS servers, which makes it straightforward for attackers to function unnoticed for lengthy intervals of time. This must be disturbing for all critical internet portals that need to shield their guests’ information, together with authentication and cost data. Organizations that use OWA also needs to listen, because it depends upon IIS and might be an attention-grabbing goal for espionage.
Whereas IIS server threats should not restricted to native IIS malware, we imagine this paper will probably be a useful place to begin for defenders for understanding, figuring out, and eradicating IIS threats, and a information to our fellow researchers to reverse engineer this class of threats and perceive their widespread techniques, strategies and procedures.
Further technical particulars on the malware and Indicators of Compromise could be present in our complete white paper, and on GitHub. For any inquiries, or to make pattern submissions associated to the topic, contact us at: threatintel@eset.com.
Acknowledgements to fellow ESET malware researchers Marc-Étienne Léveillé and Mathieu Tartare for his or her work on this investigation.
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